This invention relates to the detection and quantification of microair in blood, and more particularly to an infrared system for counting microair bubbles in a bloodstream and determining their size.
During open-heart surgery, microscopic air bubbles having a diameter on the order of 60-300 xcexcm are frequently entrained into the blood circuit of the heart-lung machine in spite of careful defoaming of the blood passing through the machine. These microair bubbles have been suspected of causing strokes, memory loss and other undesirable effects in the patient. It is therefore important in a heart-lung machine to detect the presence and size of these bubbles remaining in the bloodstream after filtration so that their origin can be traced and appropriate remedial measures can be taken when the presence of microair is detected in the blood circuit.
Microair detection in the prior art has conventionally been done by transmitting an ultrasound beam through a bloodstream flowing past the detector. The problem with this approach is that ultrasound is expensive and is neither able to accurately evaluate the size of individual bubbles, nor detect them as individual bubbles when they are close together. Also, ultrasound measures discontinuities in the bloodstream and therefore cannot distinguish between microair and tiny blood clots. Also, the accuracy of ultrasound measurements becomes poor for very small diameter bubbles. Furthermore, in pulsing ultrasound applications, the propagation velocity of sound required ultrasound pulses to be at least 10-20 xcexcsec apart for a fast-moving 1.25 cm diameter bloodstream, so that tracking is not continuous. Consequently, a more accurate and discriminating method of detecting microair was needed.
Prior to the present invention, optical detection of microair was of limited use because it only operated in a binary (bubble present or absent) mode. Quantitative detection was considered impractical because light transmission through a bloodstream is strongly affected by hematocrit and oxygen saturation, which vary unpredictably during surgery.
The present invention allows highly accurate detection and measurement of microair over a wide range of bubble sizes by detecting the translucence of a bloodstream to infrared radiation having a wavelength of about 800-850 nm in at least two directions at a substantial angle to each other. The detection is made by detecting spikes in the amplitude of infrared signals received by an array of infrared sensors disposed around a column of blood when microair bubbles pass the field of view of the sensors.